1. Field
The present invention relates to fuel cells, in particular to indirect or redox fuel cells which have applications in microfuel cells for electronic and portable electronic components, and also in larger fuel cells for the automotive industry and for stationary and other portable applications. The invention also relates to certain catholyte solutions for use in such fuel cells.
2. Description of the Related Art
Fuel cells have been known for portable applications such as automotive and portable electronics technology and stationary applications such as back-up to and uninterruptible power for very many years, although it is only in recent years that fuel cells have become of serious practical consideration. In its simplest form, a fuel cell is an electrochemical energy conversion device that converts fuel and oxidant into reaction product(s), producing electricity and heat in the process. In one example of such a cell, hydrogen is used as fuel, and air or oxygen as oxidant and the product of the reaction is water. The gases are fed respectively into catalysing, diffusion-type electrodes separated by a solid or liquid electrolyte which carries electrically charged particles between the two electrodes. In an indirect or redox fuel cell, the oxidant (and/or fuel in some cases) is not reacted directly at the electrode but instead reacts with the reduced form (oxidized form for fuel) of a redox couple to oxidise it, and this oxidised species is fed to the cathode.
There are several types of fuel cell characterized by their different electrolytes. The liquid electrolyte alkali electrolyte fuel cells have inherent disadvantages in that the electrolyte dissolves CO2 and needs to be replaced periodically. Polymer electrolyte or PEM-type cells with proton-conducting solid cell membranes are acidic and avoid this problem. However, it has proved difficult in practice to attain power outputs from such systems approaching the theoretical maximum level, due to the relatively poor electrocatalysts of the oxygen reduction reaction. In addition, expensive noble metal electrocatalysts are often used.
U.S. Pat. No. 3,152,013 discloses a gaseous fuel cell comprising a cation-selective permeable membrane, a gas permeable catalytic electrode and a second electrode, with the membrane being positioned between the electrodes and in electrical contact only with the gas permeable electrode. An aqueous catholyte is provided in contact with the second electrode and the membrane, the catholyte including an oxidant couple therein. Means are provided for supplying a fuel gas to the permeable electrode, and for supplying a gaseous oxidant to the catholyte for oxidising reduced oxidant material. The preferred catholyte and redox couple is HBr/KBr/Br2. Nitrogen oxide is disclosed as a preferred catalyst for oxygen reduction, but with the consequence that pure oxygen was required as oxidant, the use of air as oxidant requiring the venting of noxious nitrogen oxide species.
US 2006/0024539 discloses a reactor and corresponding method for producing electrical energy using a fuel cell by selectively oxidising CO at room temperature using polyoxometallate compounds.
Polyoxometallate redox couples are also disclosed in WO 2007/110663. An acknowledged problem concerning electrochemical fuel cells is that the theoretical potential of a given electrode reaction under defined conditions can be calculated but never completely attained. Imperfections in the system inevitably result in a loss of potential to some level below the theoretical potential attainable from any given reaction. Previous attempts to reduce such imperfections include the selection of catholyte additives which undergo oxidation-reduction reactions in the catholyte solution. For example, U.S. Pat. No. 3,294,588 discloses the use of quinones and dyes in this capacity. Another redox couple which has been tried is the vanadate/vanadyl couple, as disclosed in U.S. Pat. No. 3,279,949.
According to U.S. Pat. No. 3,540,933, certain advantages could be realised in electrochemical fuel cells by using the same electrolyte solution as both catholyte and anolyte. This document discloses the use of a liquid electrolyte containing more than two redox couples therein, with equilibrium potentials not more than 0.8V apart from any other redox couple in the electrolyte.
The matching of the redox potentials of different redox couples in the electrolyte solution is also considered in U.S. Pat. No. 3,360,401, which concerns the use of an intermediate electron transfer species to increase the rate of flow of electrical energy from a fuel cell.
Several types of proton exchange membrane fuel cells exist. For example, in U.S. Pat. No. 4,396,687 a fuel cell is disclosed which comprises regenerable anolyte and catholyte solutions. The anolyte solution is one which is reduced from an oxidised state to a reduced state by exposure of the anolyte solution to hydrogen. According to U.S. Pat. No. 4,396,687, preferred anolyte solutions are tungstosilicic acid (H4SiW12O40) or tungstophosphoric acid (H3PW12O40) in the presence of a catalyst.
The preferred catholyte solution of U.S. Pat. No. 4,396,687 is one which is re-oxidised from a reduced state to an oxidized state by direct exposure of the catholyte solution to oxygen. The catholyte of U.S. Pat. No. 4,396,687 includes a mediator component comprising a solution of VOSO4. The mediator functions as an electron sink which is reduced from an oxidation state of V(v) to V(IV). The catholyte also includes a catalyst for regenerating the mediator to its oxidised state, (VO2)2SO4. The catalyst present in the catholyte of U.S. Pat. No. 4,396,687 is a polyoxometallate (POM) solution, namely H5PMo12V2O40.
Besides U.S. Pat. No. 4,396,687, a number of other attempts to use oxometallate catalysts have been made. For example, in U.S. Pat. No. 5,298,343, cathode systems comprising solid metal catalysts, oxometallates and metallic acids, such as molybdic acid are disclosed.
In addition, WO 96/31912 describes the use of embedded polyoxometallates in an electrical storage device. The redox nature of the polyoxometallate is employed in conjunction with carbon electrode material to temporarily store electrons.
US 2005/0112055 discloses the use of polyoxometallates for catalysing the electrochemical generation of oxygen from water. GB 1176633 discloses a solid molybdenum oxide anode catalyst.
US 2006/0024539 discloses a reactor and a corresponding method for producing electrical energy using a fuel cell by selectively oxidising CO at room temperature using polyoxometallate compounds and transition metal compounds over metal-containing catalysts.
EP-A-0228168 discloses activated carbon electrodes which are said to have improved charge storage capacity due to the adsorption of polyoxometallate compounds onto the activated carbon.
Prior art fuel cells all suffer from one or more of the following disadvantages:
They are inefficient; they are expensive and/or expensive to assemble; they use expensive and/or environmentally unfriendly materials; they yield inadequate and/or insufficiently maintainable current densities and/or cell potentials; they are too large in their construction; they operate at too high a temperature; they produce unwanted by-products and/or pollutants and/or noxious materials; they have not found practical, commercial utility in portable applications such as automotive and portable electronics.